Doesn’t it always work like that?  If I had taken pictures step-by-step, it probably wouldn’t have turned out.  I started out by taking 1″ elastic and wrapping it around my daughter under her arms .  I cut it to length and stitched the ends together to make a big circle (I also left a little tail JUST IN CASE my measurements turned out to be off which has been known to happen).

Earlier that week, I had taken my daughter to the fabric store and let her choose 8 different colors of tulle, and then I bought one yard of each.  We also picked out a spool of ribbon.

Once home, I cut the tulle into long strips and tied them around the elastic.  It looked like a REALLY big tu-tu.  Once I had tied all of the strips (it took a few sit-down sessions), I put it on my daughter.

Instead of wearing around her waist, like a regular tu-tu, I put it around her chest (under her arms).  I used the ribbon she had picked out and tied a sash under her chest.

Once I had the sash where I wanted it and where it felt comfortable on her, I carefully put my hot glue gun under the ribbon and lightly glued the ribbon down in a few places, but not all the way around.  I wanted it to stay put but not so firmly that it didn’t have wiggle room.

I then pinned 4 pieces of ribbon (cut equal lengths) to the elastic to serve as tie-straps-sleeves.  I pinned the two front ribbons pretty close together because I like the way they looked. Once I had them pinned where I wanted them, I carefully took the tutu off of my daughter and hot glued the ribbons in place (don’t forget to take out the pins!)  
The back two I glued farther apart:

She loves the dress:

When we went out trick-or-treating, she wore a t-shirt and tights underneath the dress.

Easy, easy! The tulle cost about $1 a yard and the elastic is about the same. I think the ribbon was $2. If your kiddo is smaller than mine (she wears a size 4T or 5), you could use even less tulle.
Except for the small hand stitching on the elastic -this costume was no-sew!